My guess that why the z990 only supports LPAR mode is that quite radical changes will be required to z/OS, z/VM, Linux for zSeries or any other operating system that is going to run in basic mode. By using a hypervisor one can actually emulate the current channel implementation, so these operating systems do not need to be rewritten to take advantage of multiple LSSes. I would guess that the z990 architecture with its multiple logical channel subsystems implements an SSID other then X'0001', so one would have for LSS0 subchannel numbers starting with X'00010000', and for LSS1 subchannel number starting with X'00020000' or something. A simple modification to SIE which will swap X'0001' to X'0002' for those LPARs which are attached to the 2nd LSS. This will keep everything to do with an additional LSS shielded from the guest operating systems, and as such they will not require any major modifications.
Just guessing, but there are not that may other possibilities for this to work within the current architecture.
Jan Jaeger.
From: David Boyes <[EMAIL PROTECTED]> Reply-To: Linux on 390 Port <[EMAIL PROTECTED]> To: [EMAIL PROTECTED] Subject: Re: OT: Intel gets virtualization clue? Date: Mon, 13 Oct 2003 09:40:25 -0400
The limit of 6 preferred guests has always puzzled me -- it always seemed more a political decision (thou shalt not make LPAR look bad) than a technical decision. Was it that, or was there some serious technical problem that prohibited just marching on up through storage computing offsets until you run out of storage?
Although it's probably useful to point out that that decision is driven more by use of IFLs forcing LPAR mode than any real necessity for LPARs. It's understandable why IFLs require LPAR mode, but it's not a particularly good reason to eliminate basic mode operation. I still don't understand the z990 channel system well enough to argue that point, but I do wonder whether it's substantially more complicated than dealing with the split channel system that the 3084 and it's ilk had. We seemed to deal with that well enough w/o losing basic mode.
The psychological argument you mentioned can be continued to point out that creating a new virtual machine in a basic mode system is even less committment of resources than an LPAR requires, and with VM still trailing z/OS in some of the hardware management functions, shops running w/o z/OS really do much better operationally running in basic mode and not ever disturbing the machine configuration.
-- db
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